Size-controlled synthesis of monodispersed gold nanoparticles via carbon monoxide gas reduction

ABSTRACT

A method for forming monodispersed gold particles that includes preparing a solution of gold ions at a specific concentration and pH. Then, while stirring, dispersing CO gas into the solution. The gold ions in the solution are reduced by the CO reducing agent to form desired monodispersed gold particles. The reaction conditions are selected such that the growth period of the monodispersed gold particles is greater than a nucleation period of the gold ions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/500,376 entitled “Size-Controlled Synthesis of Monodispersed Gold Nanoparticles via Carbon Monoxide Gas Reduction,” filed Jun. 23, 2011, the disclosure of which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention is made with government support under Grant Number W81XWH-07-1-0428 awarded by the Department of Defense. The government has certain rights in the invention.

This invention is made with government support under Grant Number DGE-0940902 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

Metallic nanoparticles have attracted substantial attention due to their distinctive properties and various applications. Gold nanoparticles (AuNPs) can exhibit a strong optical response to the extinction of surface plasmons by an alternating electromagnetic field. By simply adjusting the size of the gold nanoparticles, this optical resonance can be positioned over hundreds of nanometers in wavelength across the visible into the near infrared spectrum. Since these oscillations are located on the boundary of the metal and the external medium, these waves are very sensitive to changes in this boundary, such as the absorption of molecules to the metal surface. These features render AuNPs useful as building blocks, and pave the way for fabricating biological labels, biological sensors, environmental detection of biological reagents, and clinical diagnostic technologies. Many researchers have also exploited the unique optical properties of AuNPs for biomedical applications, such as thermal ablative cancer therapy and gene therapy. Because the plasmon-derived optical resonance of gold nanoparticles is strongly related to the dimensions and morphology of the nanoparticles, the ability to synthesize monodispersed AuNPs is essential.

SUMMARY

In general, in one aspect, one or more embodiments of the invention are directed to a method for forming monodispersed gold particles. The method includes preparing a solution of gold ions. The solution is stirred while dispersing carbon monoxide (CO) gas into the solution. The CO reducing agent reduces the gold ions to form monodispersed gold particles. The reaction conditions are selected such that a growth period of the monodispersed gold particles is greater than a nucleation period of the gold ions.

In general, in one aspect, one or more embodiments of the invention are directed to a method for forming monodispersed gold particles. The method includes preparing a solution of gold ions at a specific concentration and pH. The solution is stirred while dispersing carbon monoxide (CO) gas into the solution. The CO reducing agent reduces the gold ions to form monodispersed gold particles. The reaction conditions are selected such that a growth period of the monodispersed gold particles is greater than a nucleation period of the gold ions. The concentration, pH, volume, flow rate of CO gas are selected to produce the monodispersed gold particle solution.

Other aspects of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a flow chart in accordance with one or more embodiments of the invention.

FIG. 2 shows a chart of the absorbance accordance with one or more embodiments of the invention.

FIGS. 3A-3C show charts of the absorbance and size distribution in accordance with one or more embodiments of the invention.

FIG. 4 shows a chart of an XPS measurement in accordance with one or more embodiments of the invention.

FIG. 5 shows a chart of the absorbance in accordance with one or more embodiments of the invention.

FIG. 6 shows a chart of the absorbance in accordance with one or more embodiments of the invention.

FIG. 7 shows a chart of the absorbance in accordance with one or more embodiments of the invention.

FIG. 8 shows a chart of the plasmon peak position and absorbance in accordance with one or more embodiments of the invention.

FIG. 9 shows a chart of the pH as a function of concentration of gold in accordance with one or more embodiments of the invention.

FIG. 10 shows a chart of the absorbance in accordance with one or more embodiments of the invention.

FIG. 11 shows a table in accordance with one or more embodiments of the invention.

FIG. 12 shows a chart of the absorbance in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency.

In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.

In general, embodiments of the invention are directed to forming monodispersed solid gold particles using carbon monoxide (CO). Embodiments of the invention utilizing CO as a reducing agent, to enable synthesis of AuNP with size tuning from 1 to 100 nm diameters. The size and monodispersity of the AuNPs are tunable by controlling variables such as volume, HAuCl₄ concentration, and gas flow during synthesis. The CO reduction method offered excellent tenability over a broad range of sizes while maintaining a high level of monodispersity in accordance with one or more embodiments. Ensemble extinction spectra and TEM images provide evidence that the CO reduction offer controlled AuNP tunability and CO reduction is a viable alternative to other synthesis methods.

In one or more embodiments of the invention, the gas-injection flow rates and diffuser pore size are selected based on the desired particle monodispersity. In one or more embodiments, a 60-μm average diffuser pore size is sufficient for producing monodispersed particles. Also, the solution temperature, prior to aeration, is maintained between 20 and 22° C. Those skilled in the art will appreciate that other pore sizes may be used without departing from the invention.

In one embodiment of the invention, by using CO to synthesize monodispersed gold particles, the resulting monodispersed gold particles are formed in a solution that does not include any excess reducing agent. No excess reducing agent eliminates the need for purification via multiple centrifugation steps. Further, the reduction of HAuCl₄ with CO may take place at room temperature, unlike other methods such as citrate reduction that require boiling of the solution. The time necessary to produce AuNPs using CO is less than 2 min in one or more embodiments, compared to 20 min for comparable particle sizes using citrate reduction and/or 45 min for discharge plasma synthesis. Thus, CO reduction offers a cheap and flexible alternative to current synthesis methods.

The description provides information related to AuNP synthesis, utilizing CO as a reducing agent, to enable size tuning from sub 5 to 100 nm diameters. After synthesis, AuNP mono- and polydispersity are examined. The size and monodispersity of the AuNPs are tunable by controlling variables such as HAuCl₄ concentration and gas flow during synthesis. Embodiments of the CO reduction method may provide excellent tenability over a broad range of sizes while maintaining a high level of monodispersity. Ensemble extinction spectra and TEM images provide clear evidence that CO reduction offers excellent AuNP tunability and is a viable alternative to other synthesis methods.

In one embodiment of the invention, Au³⁺ reduction by CO to Au⁰ takes place via a number of redox reactions. When the CO gas is introduced into the aqueous HAuCl₄ solution, electrons are donated to the [AuCl₄]⁻ ions. For [AuCl₄]⁻ ions to be reduced to gold atoms, a series of redox reactions take place, including the liberations of Cl⁻ ions. The above is described by Equations 1 and 2 below.

AuCl₄ ⁻+2e⁻→AuCl₂ ⁻+2Cl⁻  (1)

AuCl₂ ⁻+e⁻→Au⁰+2Cl⁻  (2)

The electrons in the above reactions may be contributed from the reaction of CO and dihydrogen monoxide and the reducing half reactions, as described by Equations 3 and 4 below.

CO(g)+H₂)→CO₂(aq)+2e⁻+2H⁺  (3)

CO(g)+2H₂O→HCO₃ ⁻+2e⁻+3H⁺  (4)

In accordance with one or more embodiments of the invention, the thermodynamics of HAuCl₄ reduction in aqueous solutions using CO is shown. The entire thermodynamic process illustrated here is performed between 20 and 22° C. and a pressure of 1 atm. The pH of the solution varies as a function of HAuCl₄ concentration. The Nernst equation (5) describes the potential of electrochemical cell as a function of concentrations of ions taking part in the reaction:

$\begin{matrix} {E = {E^{o} - {\frac{R\; T}{n\; F}{\ln (Q)}}}} & (5) \end{matrix}$

where E^(o) is the standard reduction potential, R is the absolute gas constant=8.31441 J/(mol K), F is Faraday constant=96484.6 C/mol, T is the absolute temperature=295.15 K, n is number or electrons, and Q is the reaction quotient. The ratio RT/F can be considered constant. Therefore, the reaction quotient may be expressed as:

$\begin{matrix} {Q = \frac{\left\{ C \right\}^{c}\left\{ D \right\}^{d}}{\left\{ A \right\}^{a}\left\{ B \right\}^{b}}} & (6) \end{matrix}$

Therefore, the Nernst equation may be expressed by:

$\begin{matrix} {E = {E^{o} - {\frac{R\; T}{n\; F}*2.303*{\log (Q)}}}} & (7) \end{matrix}$

In accordance with one or more embodiments of the invention, a CO gas is may be injected at a flow rate of 25.45 mL/min in a 40 mL aqueous sample volumes. A water saturation constant of 0.26 g per 1 kg at 22° C. may be used. The associated Redox potentials are given below.

Au³⁺+2e⁻→Au⁰ E⁰(V)=1.52   (8)

AuCl₄ ⁻+2e⁻→AuCl₂ ⁻+2Cl⁻ E^(o)(V)=0.926   (9)

AuCl₂ ⁻+e⁻→Au0+2Cl− Eo(V)=1.154   (10)

AuCl₄−+3e−→Au0+4Cl− Eo(V)=1.002   (11)

CO(g)+H2O→CO(aq)+2e−+2H+Eo(V)=0.11   (12)

CO(g)+2H2O→HCO3−+2e−+3H+Eo(V)=0.101   (13)

Redox potentials (11) and (12) are given at pH=0. One of ordinary skill will appreciate that the redox potentials are pH-dependent and must be adjusted for the varying pH values.

AuNPs may be synthesized by CO reduction, with average diameter ranging from 4 to 52 nm, in accordance with one or more embodiments of the invention. A set of solutions with HAuCl₄ concentrations ranging from 0.01 mM up to 2 mM may be used in the synthesis. Further, different CO flow rates may be used in the synthesis. The following is a non-exhaustive list of exemplary CO flow rates: 16.9 mL/min, 25.45 mL/min, 31.59 mL/min, 37.0 mL/min, and 42.9 mL/min. In one embodiment of the invention, the number of revolutions per minute (rpm), by which the solution is stirred, played a role in particle size and morphology. In one embodiment of the invention, the stir rate is 500 rpm.

In one embodiment of the invention, gas-injection flow rates and diffuser pore size may be varied in order to tune the amount of particles formed and the reaction completion times for forming the particles. In one or more embodiments, a 60-μm average diffuser pore size may be used to produce monodispersed particles. In one or more embodiments, the solution temperature, prior to aeration, may be maintained between 20 and 22° C.

In one embodiment of the invention, the following reagents may be used to form the monodispersed gold particles in accordance with one embodiment of the invention: (i) Hydrogen tetrachloroaurate III trihydrate (HAuCl₄ 3H2O, 99.99%), (ii) absolute ethanol (C₂H₅OH, 99.5%), (iii) Carbon monoxide (CO, 99%). Further, all solutions may be prepared using ultrapure water (18 M-ohm Millipore Milli-Q water).

In accordance with one or more embodiments of the invention, all chloroauric acid solutions are aged in individual amber bottles under 4° C. and light protected for a minimum of 3 days prior to use. All glassware used in the procedures may be cleaned in a bath of freshly prepared aqua regia solution (3 parts HCL acid to 1 part HNO₃ acid) and rinsed thoroughly with ethanol three times and then rigorously rinsed four times with copious amounts of pure grade water and oven dried prior to use. Stirring may be conducted by a PTFE-coated magnetic stir bar, which is cleaned and dried in the same manner as the glassware.

FIG. 1 is a flow chart of the basic process for producing monodisperse gold particles in accordance with one or more embodiments of the invention. In general, embodiments of the claimed invention involve preparing a solution of gold ions ST 1000 to be reduced to form monodispersed gold particles. The aqueous solution of gold ions may be derived from a metal salt, or metal complex. The concentration, pH, and volume of the gold ions in the gold ion solution may be selected based on the desired size, monodispersity, and concentration of the final solution of gold particles. The solution is stirred or mixed ST 1002 using known techniques to ensure distribution of all the species involved. Then, CO gas is dispersed ST 1004 into the solution as a reducing agent of the gold ions. The dispersal may be achieved through the use of a diffuser, or other known techniques. The rate of the dispersal of the CO gas into the gold ion solution may be selected based on the desired size, monodispersity, and concentration of the final solution of gold particles. The CO reducing agent may then reduce the gold ions ST 1006 to form the desired monodispersed gold particles.

Several chloroauric acid solutions are prepared for utilization with CO reduction in accordance with one or more embodiments of the invention. Various weights of fresh chloroauric acid may be dissolved in individual amber bottles containing water (200 mL). At least two separate batches of all solution concentrations were employed to confirm reproducibility. One set of solutions consisted of varying concentrations of chloroauric acid (0.01 to 0.09 mM in 0.01 mM increments) and HAuCl₄ (1 mM) and HAuCl₄ (2 mM) solutions are prepared. A solution of HAuCl₄ (1 wt %) is also prepared. Gold particles synthesized by CO reduction, with average diameter particles ranging from 4.5 to 52 nm are prepared as described below in accordance with one or more embodiments of the invention. For each HAuCl₄ concentration five volumes (40 mL) may be prepared. Each sample is aerated at different flow rates controlled by a control valve in accordance with one or more embodiments. The gas may enter the solution via a 60 um pore gas diffuser (Fisher Scientific) attached to the end of the gas supply line downstream of the control valve. The five solutions are exposed to CO gas at flow rates of 16.9, 25.45, 31.59, 37.0, and 42.9 mL/min, respectively. The solution temperature, prior to aeration, is maintained between 20 and 22° C.

In one or more embodiments, a 200 mL aqueous HAuCl₄ solution, with a concentration of 0.1 mM, is prepared by adding HAuCl4 of a certain weight to 200 mL of ultrapure Milli-Q water. The fresh gold solution may be used immediately or stored/aged in an amber bottle in light protected 4° C. environment for future use in accordance with one or more embodiments of the invention. A fresh stock solution of potassium carbonate (K₂CO₃ 0.5 N) is prepared and stirred for a minimum of 1 h. HAuCl₄ aqueous solutions with various pH values are prepared by the addition of certain amounts of K₂CO₃ aqueous solution into of HAuCl₄ (0.1 mM) aqueous solution (20 mL) and shaken vigorously for a minimum of 1 min.

This solution is allowed to age for 15 min before introduction of CO gas in accordance with one or more embodiments of the invention. The pH values of the aqueous solutions, measured prior to reduction, may range from 4.25 to 11.4. Additionally, several aqueous HAuCl₄ (0.38 mM) solutions (200 mL) are prepared by adding fresh gold to ultrapure Milli-Q water (200 mL). The solutions may be allowed to age for a minimum of 72 h. Potassium Carbonate, K₂CO₃, (75 mg, 2.71 mM) is added to two HAuCl₄ (0.38 mM) solutions (200 mL) and aged for 30 and 40 min, respectively. K₂CO₃ (100 mg, 3.62 mM) is added to a HAuCl₄ (0.38 mM) solution (200 mL) and aged for 30 min. All solutions are aerated with CO gas at an inlet flow rate of 25.5 mL/min.

The optical response of the gold particles is determined by examining the optical extinction spectra of aqueous samples in 1 cm path length polystyrene cuvettes using a Varian Cary 300 UV—visible spectrophotometer. The UV—visible spectra were acquired at wavelengths between 400 to 800 nm. Distilled water is used as the reference and the blank for baseline subtraction.

FIG. 2 illustrates the effects of CO gas flow injection rates on particle synthesis in accordance with one or more embodiments of the invention. Gold particles were synthesized from an aqueous solution of HAuCl₄ acid at a concentration of 0.01 mM at flow rates of 16.9 mL/min A, 25.5 mL/min B, 37.0 mL/min C, and 42.9 mL/min D. Even at this lower concentration, which may or may not be used for the synthesis of AuNPs, the extinction spectra is clearly visible and well formed as evident in FIG. 2. A smoother, more pronounced spectrum is generated at the minimum flow rate of 16.9 mL/min A when compared to the other injection flow rates.

As the flow rate is increased from 16.9 mL/min A to 42 9 mL/min D, the change in spectral symmetry is clearly visible. The gas-injection flow rate of 16.9 mL/min A produced individual particles compared to the other injection rates, as determined by TEM analysis.

In accordance with one or more embodiments of the invention, the particles produced by the 16.9 mL/min A flow rate ranged in size from 5 to 11 nm in diameter. A flow rate of 25 5 mL/min B produced particle aggregates and irregularly shaped particulate matter. Particles synthesized at a flow rate of 31.59 mL/min (not shown) consisted of aggregated particle chains. A CO flow rate of 37 mL/min C resulted in aggregated particle chains similar to that of particles produced at a flow rate of 25.45 mL/min. The particle aggregation in FIG. 2 may be evident by the broad spectral band. As the flow rate increased to 42.9 mL/min D, the particles became elliptical in shape and polydispersed. The particle sizes, when aerated at 42 9 mL/min, ranged from 5 to 40 nm in diameter with some aggregated particles. Such a size distribution may also be reflected in the broad spectral band. Those skilled in the art will appreciate that embodiments of the invention are not limited to making particles within 5 to 40 nm.

In accordance with one or more embodiments, by increasing the chloroauric acid concentration, the polydispersity of the particles may be reduced. However, the gas-injection flow rate may continue to influence the AuNP size distribution profiles.

FIGS. 3A-3C show UV—visible spectra of AuNPs synthesized from a chloroauric acid concentration of 0.03 mM at flow rates of 16.9 mL/min (FIG. 3A), 25 5 mL/min (FIG. 3B), and 37 0 mL/min (FIG. 3C) in accordance with one or more embodiments of the invention. The polydispersity of the AuNPs aerated at 16.9 mL/min (FIG. 3A) is represented by a broad particle distribution curve. The particle sizes for FIG. 3A ranged from 2.5 to 17 nm in diameter.

Sample size distributions were determined by transmission electron microscopy (TEM) performed using a JEOL 1230 High Contrast-Transmission Electron Microscope (HC-TEM) operating between 80 and 100 kV. Samples were prepared for both instruments by contacting a 10 μL drop of the AuNP solution with a carbon film coated 200 mesh copper grid. The grids were placed in a spotlessly clean container, covered and allowed to dry completely before use.

Increasing the CO flow reduced the width of the particle distribution curve where an optimum inlet gas flow is obtained at 25.5 mL/min (FIG. 3B) in accordance with one or more embodiments of the invention. The standard deviation for FIG. 3B is 7%, well below the 13% to 15% normally obtained for comparable sizes via citrate reduction.

To confirm the formation of Au atoms from HAuCl₄, the valence state of Au is identified by X-ray photoelectron spectroscopy (XPS). XPS is carried out using a PHI Quantera SXM system. The soft X-ray source consisted of aluminum with source energy of 1486.7 eV. The take off angle is set at 45°. Precut silicon wafers 4.5 mm×5 mm were cleaned by immersion in a 3:1 H₂SO₄:H₂O₂ (piranha) solution for 15 min and rinsed with ultrapure Milli-Q water and then dried. The sample is prepared by concentrating the AuNPs and dropping colloidal solution on precut silicon wafers. They were placed in a spotlessly clean container, covered and allowed to dry.

FIG. 4 shows an XPS spectrum of AuNPs synthesized via CO gas reduction in accordance with one or more embodiments of the invention. The Au 4f_(7/2) peak appeared at a binding energy of 83.98 eV and the Au 4f_(5/2) peak appeared at a binding energy of 87.71 eV. These peaks may indicate the formation of metallic gold. Solutions of particles remained stable in excess of 12 months when stored at 4° C. in accordance with one or more embodiments of the invention.

A better understanding of the effect of the gas flow rates and chloroauric acid concentrations on particle synthesis in accordance with one or more embodiments may be obtained by considering the mechanisms involved in particle nucleation and growth. When aerating an aqueous HAuCl₄ solution with CO gas, the precursor concentration increases continuously with increasing time. As the concentration reaches supersaturation, nucleation may take place and lead to a decrease in concentration. The continued decrease of the concentration is due to the growth of the particles. During the growth process, two growth mechanisms, or a combination of the two growth mechanisms, may take place. The first growth mechanism is due to the formation of particles from coalescence of the nuclei only. The second growth mechanism is due to the coalescence of nuclei into simple and multiple twins with further growth from monomer attachment of Au atoms on the surface of an existing particle.

In accordance with embodiments of the invention, to produce monodispersed AuNPs with CO gas, the rate of nucleation must be high enough so that the precursor concentration does not continue to climb. Instead, a large amount of nuclei may be formed in a short period.

When the aqueous HAuCl₄ solution is neutral or acidic, the nucleus may be formed by gold organic polymer. While the aqueous HAuCl₄ solution is alkaline, a polymerization of gold hydroxide may take place. In accordance with one or more embodiments of the invention, the rate of growth of these gold organic polymer nuclei is fast enough to rapidly decrease the concentration below the nucleation concentration. Such a method may result in the creation of a limited number of seed particles. In accordance with one or more embodiments, the rate of growth must be slow enough that the growth period is long compared with (or greater than) the nucleation period. This produces AuNPs with narrowed size distributions, which are the result of the limited nucleation period.

Because the morphology may depend strongly on injection flow rates and HAuCl₄ concentrations, a relationship between the HAuCl₄ concentration and gas-injection flow rates on particle monodispersity is used in accordance with one or more embodiments of the invention. Solution stir speeds during synthesis were examined and it is found that stir speeds had an effect on synthesis and played a role in particle size disparities. Slow solution stir speeds had the biggest affect on solutions aerated at a flow rate of 16.9 mL/min or below.

Increasing the stir speed of the solution aided in the solubility and dispersal of the CO gas molecules during synthesis. By adjusting the gas-injection flow rate, it may be possible to compensate for a reduction or increase in solution stir speed. The gas diffuser pore size may affect the synthesis process when the solution is at a standstill or stirred at a relatively slow speed (below 75 rpm). Once the solution stir speed approaches and/or crosses the 75 rpm threshold, injection-hole size may produce only small variances. Once the stir speed reaches 500 rpm, no difference between samples produced with different diffuser pore sizes is observed, and only the Au concentration or gas-injection flow rates affected particle sizes. Therefore, the solution stirring speed is maintained at 500 rpm to isolate the gas-injection flow rate and Au concentration effect on particle synthesis. One of ordinary skill in the art would appreciate that the above considerations may be related to the total volume of the solution. However, one advantage of the invention may be the ease in which embodiments of the invention may be utilized for large-scale production of AuNPs.

A chloroauric concentration of 0.03 mM and an inlet gas flow of 16.9 mL/min stirred at 500 rpm results in coalescence and growth of particles before the nucleation reached equilibrium in accordance with one or more embodiments of the invention. The induction period is initiated with a slow autocatalytic rise in the number of nuclei due to the lack of sufficient reducing agent in the solution. Because of this slow nucleus formation, new nuclei may be formed while existing nuclei may have already undergone coalescence resulting in polydispersity. Increasing the flow rate to 25.5 mL/min increases the autocatalytic rise in the number of nuclei. Particle growth may take place after cessation of the nucleation process resulting in monodispersity in accordance with one or more embodiments of the invention. The particle distribution curve shown in FIG. 3B consists of particle sizes in the range of 4 to 6 nm as opposed to the range of 2 to17 nm shown in FIG. 3A. By increasing the flow rate further, as in FIG. 3C, rapid coalescence of the nuclei may take place. The resulting polydispersity of the solution at the increased gas-injection flow rates may still be marginal compared to the lower flow rate of 16.9 mL/min. When comparing the spectra of FIGS. 3A-3C, the more polydispersed sample possesses a broadened spectrum.

Referring to FIG. 5, the effect of CO flow rate on particle spectral profile is shown in accordance with one or more embodiments of the invention. Normalized UV—visible spectra of particles synthesized from a chloroauric acid concentration of 0.03 mM aerated at flow rates of 16.9 mL/min (Spectra A), 25.5 mL/min (Spectra B), and 37.0 mL/min (Spectra C) is shown in FIG. 5. In accordance with one or more embodiments of the invention, the effect of the gas flow rate during synthesis is illustrated by a comparison of the three spectra.

In accordance with one or more embodiments of the invention, when the chloroauric acid concentration approaches 0.2 mM, the gas-injection flow rate may have a less pronounced effect on the spectra symmetry, but the flow rate continues to determine the monodispersity of the particles. When particles are synthesized from a chloroauric acid concentration of 0.3 mM, the most monodispersed sample is produced at a flow rate of 25.5 mL/min in accordance with one or more embodiments of the invention. The mean diameter for this sample is 9 nm with a standard deviation of 11%.

As the concentration increases to 0.5 mM, 20 to 25 nm particles are produced in accordance with one or more embodiments of the invention. Continual increase of the chloroauric acid concentration beyond 0.5 to 0.6 mM only produced small changes in particle size with increased absorbance. The standard deviation for the AuNPs produced at 0.6 mM is 8% indicating monodispersity. As the concentration is increased to 1 mM, particles approaching 30 nm in diameter were produced, but the standard deviation approached 20%. Further, doubling the concentration to 2 mM had no uniform effect on particle growth, with the majority of the particles in the 30 nm size regime and some of the particles in the 40 to 55 nm size regime with a standard deviation approaching 35%.

FIG. 6 is the UV—visible spectra of the above solutions prepared at different concentrations of chloroauric acid in accordance with one or more embodiments of the invention. FIG. 6 demonstrates an increase in the chloroauric acid concentration from 0.02 to 1 mM. FIG. 6 also shows an increase in absorbance with the concentration, which correlates to an increase in particle concentration and volume. FIG. 7 shows the normalized UV—visible spectra, which demonstrate the pronounced red shifting of the plasmon, which is associated with increased particle size in accordance with one or more embodiments of the invention.

FIG. 8 is a chart of the absorbance and plasmon peak position as a function of chloroauric acid concentration in accordance with one or more embodiments of the invention. The red shift of the plasmon, which may be associated with increased particle size, is illustrated in FIG. 8. In FIG. 8, the chloroauric acid concentration ranges from 0.01 mM to 1 mM, plotted on a logarithmic scale. In accordance with one or more embodiments of the invention, as the HAuCl₄ concentration increases, the absorbance intensity 802 may increase with an accompanying red-shift of the plasmon peak position. The shifting of the plasmon 804 is congruent with the prediction described by Mie theory. Statistical analysis of the particles synthesized from the aqueous solutions of HAuCl₄ ranging from 0.02 to 0.6 mM revealed an average standard deviation of approximately 11% in accordance with one or more embodiments of the invention.

One of ordinary skill in the art would appreciate that the pH is a factor influencing the nucleation and growth of AuNPs. Because the synthesis process takes place in an acidic environment, the particle may be formed from gold polymer with a small contribution from a gold hydroxide polymer reduction. As the concentration of chloroauric acid increases, the pH of the solution may decrease resulting in particle formation primarily by the gold polymer reduction.

FIG. 9 is a chart of the pH values before 906 and after 908 AuNP synthesis in accordance with one or more embodiments of the invention. In FIG. 9, pH values for HAuCl4 concentrations ranging from 0.02 to 0.1 mM in 0.01 mM increments and from 0.1 to 0.5 mM in 0.1 mM increments. The x-axis is plotted on a logarithmic scale. The inset shows the pH values of the AuNP solutions from 0.01 to 0.1 mM and is plotted on a linear scale. As the reduction of HAuCl₄ by CO takes place, H⁺ ions are liberated, decreasing the pH of the solution. All pH measurements were taken at room temperature.

In an acidic environment, an effective monodispersed particle size threshold may be reached at approximately 25 nm. The effective monodispersed threshold is defined as a standard deviation below 13%. As previously mentioned, continual increase of the chloroauric concentration may eventually result in adverse affects on particle monodispersity. To further grow particles and maintain monodispersity, HAuCl₄ hydrolysis may be used in accordance with one or more embodiments of the invention. The addition of potassium carbonate (K₂CO₃) to generate an alkaline solution for gold hydroxide polymer reduction is systematically investigated. It is found that the speciation of HAuCl₄ may influence on the size and monodispersity of the AuNPs. As the pH increased, speciation of aqueous HAuCl₄ occurred in accordance with one or more embodiments of the invention.

Adding K₂CO₃ raises the pH of the solution by allowing hydrolysis of HAuCl₄ to take place to form gold hydroxide solution. For example, a 200 mL aqueous HAuCl₄ solution, with a concentration of 0.1 mM, is prepared by adding fresh gold to 200 mL of DI water. This solution is aged in an amber bottle, and light protected in a 4° C. environment for a minimum of 72 h prior to use in accordance with embodiments of the invention. A 0.5 N stock solution of K₂CO₃ is prepared and stirred for a minimum of 1 h in accordance with embodiments of the invention. After the aging, the chloroauric acid solution is allowed to gradually rise to 22° C. The pH of the chloroauric acid solution is measured to be 3.6. HAuCl₄ (0.1 mM) aqueous solutions with various pH values were prepared by the addition of K₂CO₃ aqueous solution into 20 mL of HAuCl₄ aqueous solution and shaken vigorously for a minimum of 1 min. The solutions are subsequently allowed to age for 15 min before introduction of CO gas. The pH values of the aqueous solutions, measured prior to reduction, ranged from 4.25 to 11.4.

FIG. 10 shows UV—visible absorption spectra of AuNPs prepared by reduction of hydrolyzed HAuCl₄ at various pHs in accordance with one or more embodiments of the invention. At pH=4.25 1010, the acquired AuNPs exhibited a symmetric spectrum with a surface plasmon resonance (SPR) peak at 512 nm. When the pH increased to 6.6 1012, there is a SPR shift to 527 nm. When the pH increased to 7.45, the SPR peak position did not change much at 528 nm, and the SPR peak remained symmetric. The SPR feature changed abruptly when the pH is 9.34 1014 showing a broad feature originating at 559 nm. The SPR peak red-shifted further when as the pH increased to 10.3. Absorption in the NIR region also gained significant intensity with the higher pH values.

Previous experimental and theoretical results have demonstrated that AuCl4 undergoes a pH dependant stepwise hydrolysis which gives way to [AuCl_(x)(OH)_(4-x)]⁻. The extent of hydrolysis in turn depends on the pH, which may indicate the amount of OH⁻ available for hydrolysis. When the pH is low, [AuCl₄]⁻ ions may dominate the solution. As the pH is increased to 4.25, [AuCl₄]⁻ concentration is lowered and the contribution from [AuCl₃(OH)]⁻ ions may be increased. Raising the pH of the solution to 6.66 may reduce the concentration of [AuCl₄]⁻ and [AuCl₃(OH)]⁻ significantly, and the ionic composition may be primarily made up of [AuCl₂(OH)₂]⁻ ions. Further increasing the pH to 8.8 may result in a large ion contribution from [AuCl(OH)₃]⁻ ions. Additional increase of the pH to 10.3 may result in an overwhelming ion contribution from [Au(OH)₄]⁻ ions with an appreciable contribution from [AuCl(OH)₃]⁻ ions. This may be due to [Au(OH)₄]⁻ being amphoteric. Its solubility may be increased due to the formation of [Au(OH)₄]⁻ at a higher pH, thus making the soluble [Au(OH)₄]⁻ the dominant species at high pH, instead of the precipitating [AuCl(OH)₃]⁻. It is the control of hydrolysis to tune the speciation of [AuCl_(x)(OH)_(4-x)]⁻ that subsequently influences the particle size.

It is observed that amongst the six species of [AuCl_(x)(OH)_(4-x)]⁻ discussed earlier, [Au(OH)₄]⁻ appears to have a lower tendency to be reduced in solution to form colloidal gold. This is evident from a slow and gradual color change during reduction, taking approximately 7 min for complete reduction to occur. In contrast, the reduction of other [AuCl_(x)(OH)_(4-x)]⁻ species formed at a lower pH where it is observed that the addition of CO gas caused a color change of the solution within seconds, and total reduction within approximately 2 min. This observation may be attributed to a weaker reduction potential of [Au(OH)₄]⁻ compared to other species. In accordance with one or more embodiments of the invention, adjustment of the pH to a pH<10 by addition of small amounts of K₂CO₃ may result in the formation of other dominant species that has a greater tendency to be reduced in solution to form colloidal gold.

One of ordinary skill in the art will appreciate that the synthesis environment may also affect particle stability. As the pH increased, prior to synthesis, the particles became less stable in accordance with one or more embodiments of the invention. FIG. 11 shows a illustrating the stability of the AuNP solutions produced at varying pH in accordance with one or more embodiments of the invention.

In accordance with FIG. 11, the hydrolysis of [AuCl₄]⁻ starts to occur within minutes after the addition of K₂CO₃, indicating immediate formation of the [AuCl_(x)(OH)_(4-x)]⁻ species. Au colloid, of varying sizes, is produced when K₂CO₃ and HAuCl₄ concentrations and gas-injection flow rates remained constant and only the aging times varied. This may indicate that the aging the gold hydroxide solution, before the addition of CO gas, has a strong influence on the outcome of the reaction in accordance with one or more embodiments.

In one or more embodiments of the invention, by controlling the development of the [AuCl_(x)(OH)_(4-x)]⁻ species, colloids of various sizes may be synthesized using CO as a reducing agent. When the pH is sufficiently high, the resultant aging process may generate coalescence of Au(OH)₄, initiating a limited nucleation process absent of a reducing agent. However, such a nucleation process is out of favor with the requirements necessary for generating monodispersed particles. Thus, in accordance with one or more embodiments of the invention, proper aging times may need to be determined in order to synthesize monodispersed particles of a particular size from a given K₂CO₃ and HAuCl₄ concentration. Embodiments of the invention may exploit the control of [AuCl_(x)(OH)₄]⁻ species development by the addition of K₂CO₃ and aging of the solution. AuNPs in the ranges of 15 to 100 nm in diameter may be produced.

FIG. 12 shows the UV—visible spectra of Au colloid produced in accordance with one or more embodiments of the invention. Spectra A and B in FIG. 11 show the UV—visible spectra of Au colloid produced from a mixture of 200 mL of a 0.38 mM HAuCl₄ aqueous solution and 2.71 mM K₂CO₃ aged at 30 and 40 min, respectively. The solution reduction volumes were 40 mL. Both plasmon resonance peaks were well ordered with a peak at 536 nm for the 30-min aged solution (Spectra A) and 546 nm for the 40-min aged solution (Spectra B). Both solutions were aerated with CO gas at an inlet gas flow rate of 25.5 mL/min. The red-shift and dampening of the SPR peak may be an indication of an increase in particle size.

In accordance with one or more embodiments of the invention, the solution volume being aerated may affect the particle size and monodispersity. Spectra C, D, and E in FIG. 12 were produced from AuNPs synthesized from a 200 mL 0.38 mM HAuCl₄ aqueous solution with 3.62 mM K₂CO₃ aged for 30 min. The aeration volumes were 20 mL (Spectra C), 40 mL (Spectra D), and 50 mL (Spectra E). The amount of solution aerated had a small but noticeable effect on plasmon peak position. The resulting plasmon peak positions were 550nm (Spectra C), 553 nm (Spectra D), and 554 nm (Spectra E). In accordance with one or more embodiments of the invention, increasing the amount of K₂CO₃, in a HAuCl₄ aqueous solution of known concentration, while decreasing the aging time, may produce larger AuNPs, while still maintaining the desired monodispersity. Aqueous solutions of 200 mL 0.38 mM HAuCl₄ with 2.71 and 3.62 mM of K₂CO₃ aged for 30 min (Spectra A and C, respectively) each produced AuNPs with SPR peak positions at 536 nm and 553 nm, respectively.

In accordance with one or more embodiments of the invention, employing a combination of gold polymer reduction and/or gold hydrolyzed polymer reduction, monodispersed particles sizes from ˜4 nm to 100 nm may be synthesized. Monodispersed solid gold particles greater than 100 nm may be synthesized in accordance with the methods disclosed herein by adjusting the hydrolysis of the gold species prior to reduction with CO. TEM analysis of solutions of AuNPs synthesized without the addition of K₂CO₃, and AuNPs synthesized from a hydrolyzed solution of aqueous HAuCl₄ via the addition of K₂CO₃ resulted in sizes of AuNPs of 4 nm, 6 nm, 15 nm, 25 nm, 50 nm, and ˜100 nm with standard deviations of 7%, 13%, 8%, 8%, 10%, and 11%, respectively.

Embodiments of the invention include AuNPs synthesized using CO as a reducing agent. CO as a reducing agent offers tunability of particle sizes via altering HAuCl₄ concentration and flow rate. Advantages of the invention include fast synthesis rates, ease of tunability, and absence of byproducts may allow for CO-based AuNPs to be optimized and readily produced for biomedical and industrial applications. In one or more embodiments, the manipulation of the solution pH and speciation of HAuCl₄ may be used to control particle morphology and also as a means to tune the particle size. TEM micrographs and UV—visible spectral analysis have confirmed that the CO-based AuNPs are monodispersed upon synthesis.

Further, as compared to the current synthesis methods, CO has an advantage in that no excess reducing agent remains in solution. This may eliminate the need for further purification. The reduction of HAuCl₄ with CO may also take place at room temperature, unlike other methods such as citrate reduction that require boiling of the solution. The time necessary to produce AuNPs using CO may be less than 2 min compared to 20 min for comparable particle sizes using citrate reduction and 45 min for discharge plasma synthesis. CO reduction may offer a cheap and flexible alternative to femtosecond laser-based AuNP synthesis processes, while eliminating the need for surfactants and polymers to tune the particle sizes.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

What is claimed is:
 1. A method for forming solid monodispersed gold particles, comprising: preparing a solution of gold ions; dispersing CO gas into the solution; and reducing, using the CO gas, the gold ions to form the monodispersed solid gold particles, wherein a growth period of the monodispersed solid gold particles is greater than a nucleation period of the gold ions.
 2. The method of claim 1, wherein the average diameter of the monodispersed solid gold particles ranges from 4 nm to 52 nm.
 3. The method of claim 2, wherein the CO gas is dispersed using a diffuser.
 4. The method of claim 3, wherein the diffuser has an average pore size of 60-μm.
 5. The method of claim 1, wherein the CO gas is dispersed in the solution of gold ions at a flow rate of at least 16 mL/min.
 6. The method of claim 1, wherein the CO gas is dispersed in the solution of gold ions at a flow rate of at least 42 mL/min.
 7. The method of claim 1, wherein the gold ions solution is a chloroauric acid solution
 8. The method of claim 1, wherein a concentration of gold ions in the solution of gold ions is between 0.01mM and 1 mM.
 9. The method of claim 1, wherein the solution of gold ions is stirred using a stir bar rotating at an RPM of at least
 75. 10. The method of claim 1, wherein the polydispersity of the monodispersed gold particles is less than 13%.
 11. A method forming monodispersed solid gold particles comprising: preparing a solution of gold ions with a specific pH; dispersing CO gas into the solution; and reducing, using the CO gas, the gold ions to form the monodispersed solid gold particles, wherein a growth period of the monodispersed solid gold particles is greater than a nucleation period of the gold ions.
 12. The method of claim 11, wherein the specific pH of the solution of gold ions is set by the addition of potassium carbonate.
 13. The method of claim 11, wherein the CO gas is dispersed using a diffuser with an average pore size of 60-μm.
 14. The method of claim 11, wherein the CO gas is dispersed in the solution of gold ions at a flow rate of at least 16 mL/min.
 15. The method of claim 11, wherein the CO gas is dispersed in the solution of gold ions at a flow rate of at least 42 mL/min.
 16. The method of claim 11, wherein the gold ions solution is a chloroauric acid solution
 17. The method of claim 11, wherein the concentration of gold ions in the solution of gold ions is between 0.01mM and 1 mM.
 18. The method of claim 11, wherein the solution of gold ions is stirred using a stir bar rotating at an RPM of at least
 75. 19. The method of claim 11, wherein the polydispersity of the monodispersed solid gold particles is less than 13%.
 20. The method of claim 11, wherein the solution of gold ions is aged for at least 72 hrs prior to the dispersing of the CO gas. 